DEVELOPMENT OF EXCRETORY SYSTEM 781 



nephric duct as shown in figure 345C-1 to C-5. A nephrostomal connection 

 with the coelomic cavity also appears, but the nephrostomal segment soon 

 acquires a secondary connection with a renal vein (fig. 345C, 4-6). The 

 veins thus come to drain the coelomic cavity directly. (In the water-abiding 

 urodele, Necturus maciilosus, the nephrostomal connection remains in con- 

 tact with some of the renal units, even in the adult. See figure 345D.) 



As the mesonephric kidney of the frog continues to develop, many new 

 mesonephric renal units are added, and several units appear in each body 

 segment. In consequence the primitive segmental arrangement of the renal 

 units is lost, particularly in the caudal region of the nephrotomic plate where 

 the kidney is developed most highly. Collecting ducts develop as evaginations 

 of the mesonephric duct and the renal units discharge their contents into these 

 collecting ducts. 



Caudally situated nephrotomic material, comparable to the metanephric 

 area of the kidney of higher vertebrates, is incorporated along with the meso- 

 nephric kidney as in the shark embryo. The adult form of the kidney, there- 

 fore, may be regarded as an opisthonephros, composed of mesonephric and 

 metanephric renal units. 



c. Chick 



The mesonephros of the chick develops from the nephrotomic plate in the 

 region between the somites 13 and 30. The nephrotomic plate in the chick 

 embryo increases its substance rapidly through cell proliferation posterior to 

 the area of pronephric-kidney origin. The original nephrotomic plate in this 

 way becomes converted into an elongated mass or cord of cells called the 

 nephrogenic cord. The mesonephric tubules arise as condensations within 

 this cord of nephrogenous tissue. The renal unit emerges initially as a rounded 

 mass of epithelial cells as in the frog. These epithelial masses elongate. They 

 acquire a Malpighian body at one end, while the other end unites with the 

 mesonephric duct. Some of the anterior tubules may have coelomic connec- 

 tions, similar to the pronephric tubules, but as this portion of the meso- 

 nephric kidney degenerates, these nephrostomal structures have little func- 

 tional significance. 



As development progresses, the nephrotomic substance increases greatly 

 through proliferation of its constituent cells, and several renal units arise in 

 each body segment (fig. 345F). To aid this process, the mesonephric duct 

 forms collecting ducts which extend outward into the region of the developing 

 renal units, and a group of these units joins each collecting duct (fig. 345G). 

 The mature form of the mesonephric tubule of the chick consists of a 

 glandular (secretory) segment which connects with either the mesonephric or 

 the collecting duct on the one hand and with a Malpighian body and its 

 glomerulus on the other (fig. 345G). The mesonephric kidney of the chick 

 is a prominent excretory organ from the fifth to the eleventh day. During 



